1. Field of the Invention
The invention relates to broadband antennas for use in mobile devices, such as cellular telephones and other wireless communication devices. More particularly, the invention relates to broadband antennas systems having improved bandwidth and efficiency over conventional broadband antennas used in mobile devices.
2. Description of the Related Art
Mobile communication devices, such as cellular telephones and other portable communication devices, each require some sort of antenna to establish and maintain a wireless radio link with another unit in the system, usually a wireless base station. In general, antennas (in transmit mode) generally convert radio frequency electrical currents into electromagnetic waves and (in receive mode) convert electromagnetic waves into radio frequency electrical currents. As mobile communication devices become smaller in size, the resulting space limitations have made it more difficult to design and implement antennas that are sufficiently efficient for proper and improved mobile communication device operation.
Several different types of antennas can be used in a mobile communication device. For example, a slot antenna includes a radiator formed by cutting a narrow slot in a large metal surface. The slot length is a half wavelength at the desired frequency and the width is a small fraction of a wavelength. Another antenna often used in mobile communication devices is a microstrip antenna or patch antenna. Patch antennas use a conductive material that is formed in a stripline, rectangular or other shape, and disposed on a dielectric substrate having a certain dielectric value and thickness. The shape of the conductor is chosen to achieve the desired resonant frequency and radiation pattern. Patch antennas offer relatively large degree of flexibility in antenna and wireless-device design, as they are cost-effective, easily manufactured, and can be conformed to the shape of a mobile communication device.
A derivation of the patch antenna is a planar inverted F antenna, or PIFA. Compared to a conventional patch antenna, the PIFA can resonate at a much smaller patch size for a fixed operating frequency. A conventional PIFA structure includes a conductive radiator element disposed parallel to a ground plane and insulated from the ground plane by a dielectric material, usually air. The radiator element is connected to two pins, typically disposed toward one end of the element, thus giving the appearance of an inverted letter “F” from the side view. One pin electrically connects the radiator to the ground plane; the other pin provides the antenna feed. Impedance matching is obtained by selecting correct positioning of the feed and ground contacts. Accordingly, a conventional PIFA structure is similar to a shorted rectangular microstrip patch antenna.
However, as mobile communication devices become smaller in size, conventional antennas often are too large to fit within the mobile communication device. Also, next generation mobile communication devices include operating ranges that are beyond the most efficient operating regions of conventional mobile communication device antennas. Therefore, a need exists for antennas that are small enough to fit in current and future mobile communication devices, yet still provide sufficient and even better bandwidth, multi-band operation and operating efficiency despite their reduced size.